Barack Obama will fly to Louisiana today to assess the threat to large stretches of the Gulf of Mexico from millions of gallons of crude oil that has spilled from the remains of a sunken offshore rig. One expert said that the slick had tripled in size in a day.

Satellite images found the surface area of the spill from the Deepwater Horizon rig, which exploded on 20 April with the loss of 11 lives, had increased from about 1,150 sq miles on Thursday to around 3,850 sq miles by the end of Friday.

“The spill and the spreading is getting so much faster and expanding much quicker than estimated,” said Hans Graber from Miami University, which analysed the images. Sea currents could suck the sheen up the eastern coast, another expert said. But the head of an assessment team for oil spills cautioned that the imagery may be deceptive as satellites could not measure the thickness of the sheen.

BP’s analysis for the well had concluded that it was virtually impossiblefor there to be a giant crude oil spill, it was revealed yesterday.

Efforts to stem the flow from the sea floor and remove oil from the surface by skimming it, burning it or by dispersing it with chemicals continued with little success. Adding to the gloomy outlook were warnings from experts that an uncontrolled gusher could create a nightmare scenario if the Gulf Stream current carries it toward the Atlantic.

The Deepwater Horizon well is at the end of one branch of the Gulf Stream, the famed warm-water current that flows from the Gulf of Mexico to the north Atlantic. Several experts said that if the oil entered the stream, it would flow around the southern tip of Florida and up the eastern seaboard.

“It will be on the east coast of Florida in almost no time,” Graber said. “I don’t think we can prevent that. It’s more of a question of when rather than if.”

There is growing criticism that the government and BP should have done more to stave off the disaster, which cast a pall over the fragile environment and the region’s economy, still recovering from the devastation of 2005’s Hurricane Katrina.

The coastguard conceded yesterday that it was nearly impossible to know how much oil has gushed since the explosion, after saying earlier it was at least 1.6m gallons (6m litres) – equivalent to about 2½ Olympic-sized swimming pools. Even at that rate, the spill should eclipse the 1989 Exxon Valdez incident as the worst US oil disaster in history in a matter of weeks.

Doug Suttles, BP’s chief operating officer for exploration and production, said the company and federal officials were preparing for the worst-case scenario.

He was reluctant to describe what, exactly, a worst-case scenario would look like but if the oil gets into the Gulf Stream and is carried to the beaches of Florida, it stands to be an environmental and economic disaster of epic proportions.

From land, the scope of the crisis was difficult to see. As of Saturday afternoon, only a light sheen of oil had washed ashore in some places.

The real threat lurked offshore in a swelling, churning slick of dense, rust-coloured oil the size of Puerto Rico. From the endless salt marshes of Louisiana to the white-sand beaches of Florida, there is uncertainty and frustration over how the crisis got to this point and what will unfold in the coming days, weeks and months.

BP has not said how much oil is beneath the Gulf seabed Deepwater Horizon was tapping, but a company official speaking on condition of anonymity confirmed reports that it was tens of millions of barrels.